This invention is generally directed to developer compositions which can be utilized for the development of images in electrostatographic imaging systems. More specifically, the present invention is directed to developer compositions containing certain quaternized vinyl pyridine polymers, as charge enhancing additives. The quaternized vinyl pyridine additives of the present invention impart a positive charge to the toner particles, enabling such particles to be utilized for the development of negatively charged electrostatic latent images.
The electrostatic process, and more specifically, the xerographic process is well known as documented in several prior art references. These processes involve the application of toner particles to the electrostatic latent image, for the purpose of causing development of such images. In some instances it may be desirable to produce a negative copy from a positive original, or a positive copy from a negative original, known as image reversal. In electrostatic imaging systems, image reversal can be accomplished by applying to the image a developer composition, containing toner particles and carrier particles, wherein the toner particles are repelled by the charged areas of the image, but adhere to the discharged areas. More specifically, toner particles possessing positive charges are very useful and effective in electrostatographic reversal systems, especially those employing organic photoreceptors, which in many instances are charged negatively. Reversal developers, which are comprised of electroscopic materials coated with finely divided colloidal silica are described for example, in U.S. Pat. No. 2,986,521.
Several prior art references disclose the use of charge control agents, such as specific quaternary ammonium compound, which agents impart a positive charge to the toner particles. For example, in U.S. Pat. No. 3,893,935, there is disclosed the use of certain quaternary ammonium salts as charge control agents for electrostatic toner compositions. According to the disclosure of this patent, certain quaternary ammonium salts when incorporated into toner materials enabled the provision of a toner composition which exhibited relatively high uniform and stable net toner charge, when mixed with a suitable carrier material. U.S. Pat. No. 4,079,014 contains a similar teaching, with the exception that a different charge control agent is used, namely a diazo type compound.
Many of the above described toners and developers have a tendency over a period of time to lose their positive charge. Also in some instances the charge control additives employed are incompatible with the thermoplastic toner resin, thus causing difficulties in obtaining a uniform dispersion of such additives in the toner resin. Further as some of the prior art charge control agents are not compatible with the toner resin, they adversely affect the electrical properties of the resin. Additionally, primarily because of their low molecular weight, such charge control agents have been known to leach out of the toner composition, and contaminate the carrier surface. Accordingly, thus there is a need for toner and developer compositions which can be used in a reversal system, and more specifically, there is a need for positively charged dry electrostatographic toner materials, which will enable the production of high quality images over extended periods of time.